The present invention relates to an intake device of a multi-cylinder internal combustion engine.
As an internal combustion engine for obtaining a high output, there is known a multi-cylinder internal combustion engine in which each cylinder is provided with a first intake port for low-speed driving having a first intake valve for low-speed driving and a second intake port for high-speed driving having a second intake valve for high-speed driving. Said first intake port and said second intake port of each cylinder communicate with ambient air through a common first throttle valve, and a normally closed second throttle valve is arranged in the second intake port of each cylinder downstream of the first throttle valve so that when the engine load or engine rotation number exceeds a predetermined value, said second throttle valve opens. In this multi-cylinder internal combustion engine, at the time of low or medium engine load driving or low or medium engine speed driving, the second throttle valve closes and, therefore, air is supplied into the engine cylinder only through the first intake valve. On the other hand, at the time of high engine load driving or high engine speed driving, since the second throttle valve opens, air is supplied into the engine cylinder through the first intake valve and second intake valve. In the internal combustion engine of this type, since air is supplied into the engine cylinder through the first and second intake valves at the time of high engine load driving or high engine speed driving as described above, the volumetric efficiency is increased and there can be obtained a higher output than the output obtainable in the case where only one intake valve is arranged. However, at the time of low or medium engine load driving where only the first intake valve is open, the degree of turbulence produced in the engine cylinder is low and the burning velocity is low, with the result that stable combustion cannot be obtained and specific fuel consumption is increased. Moreover, there arises a problem of unstable ignition. For solving these problems, there has been proposed an internal combustion engine in whrch a helical type intake port is used as the first intake port, or a shroud is formed for the first intake valve to produce a strong swirling stream in the combustion chamber. However, such helical intake port or shroud valve offers a great degree of resistance to air, and, therefore, the volumetric efficiency is reduced at the time of high engine load driving or high engine speed driving, with the result that a high output cannot be obtained.